1. Field of the Invention
The invention relates to computer systems, and more particularly to power consumption of computer systems.
2. Description of the Related Art
A computer system changes power consumption of a central processing unit (CPU) thereof according to its operating status. When computer load is reduced, the computer system lowers power consumption of the CPU for power saving. For example, an operating frequency of the CPU is reduced, and an operating voltage of the CPU is also lowered, thus reducing power consumption of the CPU. On the contrary, when computer load is increased, the computer system increases power consumption of the CPU, thus allowing the CPU to process more computing tasks during a predetermined period.
For example, Advanced Configuration and Power Interface (ACPI) is a standard for computer systems. In the ACPI standard, power consumption of a CPU is divided into five statuses including a C0, C1, C2, C3, and C4 status. The C0 status is an active status in which power consumption of a CPU is the highest. The C1 status is a halt status in which a CPU consumes less power than that during the C0 status. The C2 status is a stop status, and the C3 status is a sleep status, wherein the power consumption of a CPU is even further reduced. Finally, the C4 status is a deep-sleep status in which power consumption of a CPU is the lowest. Thus, a computer system dynamically adjusts the power consumption status of a CPU thereof, according to its operating status.
Referring to FIG. 1, a schematic diagram of switching of a power consumption status of a CPU according to the ACPI standard is shown. To begin, a computer operating system continues to evaluate an operating status of its CPU to determine whether to change a power consumption status of the CPU. In one embodiment, a Basic Input and Output System (BIOS) of the computer system evaluates the operating status of the CPU to determine whether to change the power consumption status of the CPU.
Peripheral devices are ordinarily coupled to a CPU via a control device. For example, a Universal Serial Bus (USB) device is coupled to a CPU via a control device such as a chipset. The control device includes a USB controller for handling data transmission between the USB device and the CPU. When the CPU requires data transmission with the USB device, the CPU stores information about data transmission in a descriptor in a memory coupled to the control device. The USB device reads the descriptor from the memory to understand information about the data transmission requested by the CPU. The USB device performs data transmission according to the descriptor.
The USB device therefore accesses the memory to read the descriptor to know whether data transmission between the CPU and the USB device has been requested. In addition, the USB device accesses the memory in a high enough frequency to ensure a short delay of data transmission. When the USB device accesses the memory at a high frequency, the control device, however, continues to operate even during an idle state, prohibiting the CPU from entering a C3 sleep status or a C4 deep-sleep status. Thus, when a USB device is connected to the computer system, power consumption of the CPU is prevented from being reduced. As such, the conventional method for reducing power consumption of a computer system is deficient. Thus, another method for reducing power consumption of a computer system is therefore required.